The present invention relates to a stepping motor, which uses a permanent magnet as a rotor.
As illustrated in FIG. 5, a basic stepping motor 100 is configured such that a rotor 101 composed of a permanent magnet and stator 102a, 102b is arranged around an outer circumference of the rotor 101; the rotor 101 is rotatably supported by bearing 104 via a rotor shaft 101a. 
The stator 102a, 102b has stator yoke 103a, 103b which has claw-pole-type pole teeth, and drive coil 109 stored inside the stator yoke 103a, 103b. 
More specifically described, the stator yoke 103a, 103b consists of an inner yoke 106 and an outer yoke. The inner yoke 106 has a plurality of pole teeth, which are bent at a right angle at ring-like flange portion 105. Note that the outer yoke has the same structure as that of the inner yoke, and its description is omitted.
Then, the inner yoke 106 and the outer yoke are arranged such that the pole teeth thereof are alternately arranged between the pole teeth of the other. The drive coil 109 is stored in a space created between the stator yoke 103a (103b) and a case 110. Note that the stator yoke 103a, 103b is generally formed as a member separate from the bearing 104.
Since the rotor 101 is placed on the inner side of the coil 109, the inner diameter of the coil 109 cannot be smaller than the outer diameter of the rotor 101. Thus, this prevents having a smaller configuration.
As a stepping motor 200 illustrated in FIG. 6, a motor to be used in a camera is configured such that stators 201a and 201b are arranged in the axial direction sandwiching a rotor 202 therebetween. Thus, the configuration of this type of motor is smaller in size than that of the above mentioned basic stepping motor, and also a large outer diameter can be given to the rotor 202, which is a benefit to the output.
In other words, stator yoke 208a, 208b is configured in the following manner: an outer yoke 204, which has a comb-like outer side, and an inner yoke 207 having a comb-like shaped larger diameter portion 205, which has the same diameter as that of the outer yoke 204, and a smaller diameter portion 206 are put together, and these are provided in pairs; the comb-like portion of the outer yoke 204 and the comb-like portion of the inner yoke 207 are arranged alternately adjacent to each other around a circumference; and the outer yoke 204 and the smaller diameter portion 206 of the inner yoke are opposite to each other in the radial direction. With this, the coil 209 and the rotor 202 can be arranged so as not to overlap with each other in the radial direction. This makes the outer diameter of the motor small.
With the configuration illustrated in FIG. 6, however, pressing of the stator yoke 208a, 208 is very difficult and therefore it is difficult to obtain a consistent shape. Also, a magnetic flux concentrates and saturates at the inner yoke smaller diameter portion 206 around a rotor shaft 210 because the rotor shaft 210 has a small cross-section. This causes a magnetic flux distribution having an area that does not contribute to the torque, and a generated magnetomotive force is consumed in this area. Thus, the performance cannot be improved. The area that does not contribute to torque may be made thicker than other areas to enlarge the cross-section thereof in order to improve efficiency of the motor. However, it is not easy to form that area such that it is large by pressing.
Thus, an objective of the present invention is that, by using a sintered material for a center portion through which a rotor shaft passes, the thickness of a pole core (a cross-section of the stator yoke) can be freely adjusted to prevent the eddy-current loss that is caused by magnetic saturation during high speed rotation, in order to improve motor properties. The sintered material used for the center portion also has a bearing function in order to reduce the number of components. Also, the concentric positioning of the pole cores may be facilitated in order to improve operability.
To achieve the above objective, the present invention provides a stepping motor in which a pair of drive coils are arranged in the axial direction to sandwich a magnetic rotor, wherein the drive coils are configured to be annular having a rotor shaft as a center, the rotor shaft extending from the magnetic rotor in the both directions, a pair of cylindrical yokes are provided to enclose the drive coils, a pair of sintered bearings that support the movement of the rotor shaft are made of a magnetic material, and the cylindrical yokes and the sintered bearings are integrally formed.
Each of the sintered bearing is of a cylindrical shape with an inner wall in the center that the rotor shaft moves with respect to, and is positioned between an inner wall of the drive coil and the rotor shaft. Also, each of the sintered bearing is configured such that the cylindrical end portion thereof located at the outer end of the drive coil is enlarged to form a disk-like flange that is a portion of the stator yoke, together with the cylindrical portion.
In another aspect of the invention, a stepping motor comprises a rotor shaft, a magnetic rotor fixedly disposed around the rotor shaft, a pair of drive coils disposed annularly with the rotor shaft and disposed to sandwich the magnetic rotor in the axial direction of the rotor shaft, a pair of cylindrical yokes surrounding the pair of drive coils, and a pair of sintered bearings made of magnetic materials and supporting the rotational movement of the rotor shaft.
In another aspect of the invention, a stepping motor comprises a rotor shaft, a rotor having a permanent magnet and being disposed around the rotor shaft, stators disposed to sandwich the rotor in the axial direction of the rotor shaft and surround drive coils, and bearings rotatably supporting the rotor. A portion of the stators arranged to surround the drive coils are made of a sintered magnetic material.